Devices fabricated from silicon carbide are typically passivated with an oxide layer, such as SiO2, to protect the exposed SiC surfaces of the device and/or for other reasons. However, the interface between SiC and SiO2 may be insufficient to obtain a high surface mobility of electrons. More specifically, the interface between SiC and SiO2 conventionally exhibits a high density of interface states, which may reduce surface electron mobility.
Recently, annealing of a thermal oxide in a nitric oxide (NO) ambient has shown promise in a planar 4H-SiC MOSFET structure not requiring a p-well implant. See M. K. Das, L. A. Lipkin, J. W. Palmour, G. Y. Chung, J. R. Williams, K. McDonald, and L. C. Feldman, “High Mobility 4H-SiC Inversion Mode MOSFETs Using Thermally Grown, NO Annealed SiO2,” IEEE Device Research Conference, Denver, Colo., June 19-21, 2000 and G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, R. A. Weller, S. T. Pantelides, L. C. Feldman, M. K. Das, and J. W. Palmour, “Improved Inversion Channel Mobility for 4H-SiC MOSFETs Following High Temperature Anneals in Nitric Oxide,” IEEE Electron Device Letters accepted for publication, the disclosures of which are incorporated by reference as if set forth fully herein. This anneal is shown to significantly reduce the interface state density near the conduction band edge. G. Y. Chung, C. C. Tin, J. R. Williams, K. McDonald, M. Di Ventra, S. T. Pantelides, L. C. Feldman, and R. A. Weller, “Effect of nitric oxide annealing on the interface trap densities near the band edges in the 4H polytype of silicon carbide,” Applied Physics Letters, Vol. 76, No. 13, pp. 1713-1715, Mar. 2000, the disclosure of which is incorporated herein as if set forth fully. High electron mobility (35-95 cm2/Vs) is obtained in the surface inversion layer due to the improved MOS interface.
Unfortunately, NO is a health hazard having a National Fire Protection Association (NFPA) health danger rating of 3, and the equipment in which post-oxidation anneals are typically performed is open to the atmosphere of the cleanroom. They are often exhausted, but the danger of exceeding a safe level of NO contamination in the room is not negligible.
Growing the oxide in N2O is possible as described in J. P. Xu, P. T. Lai, C. L. Chan, B. Li, and Y. C. Cheng, “Improved Performance and Reliability of N2O-Grown Oxynitride on 6H-SiC,” IEEE Electron Device Letters, Vol. 21, No. 6, pp. 298-300, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. Xu et al. describe oxidizing SiC at 1100° C. for 360 minutes in a pure N2O ambient and annealing in N2 for 1 hour at 1100° C.
Post-growth nitridation of the oxide on 6H-SiC in N2O at a temperature of 1100° C. has also been investigated by Lai et al. P. T. Lai, Supratic Chakraborty, C. L. Chan, and Y. C. Cheng, “Effects of nitridation and annealing on interface properties of thermally oxidized SiO2/SiC metal-oxide-semiconductor system,” Applied Physics Letters, Vol. 76, No. 25, pp. 3744-3746, June 2000, the disclosure of which is incorporated by reference as if set forth fully herein. However, Lai et al. concluded that such treatment deteriorates the interface quality which may be improved with a subsequent wet or dry anneal in O2 which may repair the damage induced by nitridation in N2O. Moreover, even with a subsequent O2 anneal, Lai et al. did not see any significant reduction in interface state density as compared to the case without nitridation in N2O.
In addition to NO and N2O growth and annealing, research has also been conducted on post growth anneals in other environments. For example, Suzuki et al. investigated post oxidation annealing in hydrogen. Suzuki et al., “Effect of Post-oxidation-annealing in Hydrogen on SiO2/4H-SiC. Interface,” Material Science Forum, Vols. 338-342, pp. 1073-1076, 2000. These researchers reported that flat-band voltage shift and interface state density could be improved by post oxidation annealing in both argon and hydrogen. In this research, 4H-SiC was oxidized in dry O2 at 1200° C. Post oxidation annealing was then carried out in argon or hydrogen for 30 minutes at 400°, 700°, 800° and 1000° C. Other researchers, however, have reported that post oxidation anneals in hydrogen provide no increased benefit over post oxidation anneals in other gases. Mrinal Das, “Fundamental Studies of the Silicon Carbide MOS Structure,” Doctoral Thesis, Purdue University, submitted December, 1999.